In the field of radio frequency (RF) power amplifiers (PAs), a PA is typically designed to be ‘matched’ into a 50 ohm load impedance, to ensure efficient power transfer from an RF input signal to an amplified RF output signal. This enables a low power RF input signal to be amplified and a maximum amount of the amplified signal forwarded on to, say, an antenna switch and/or an antenna. In this manner, maximum power transfer is achieved and minimal power is reflected back into the PA output. The reflection back of power is typically due to “load mismatch”, for example where the antenna switch or antenna load does not exhibit a 50 Ohm load. This can be due to the antenna being located near an object that affects its radiation properties, and correspondingly its impedance values.
It has been found that load mismatch problems occur, in particular, under both high power conditions and when a high battery voltage is applied to the power amplifier. In this regard, it can be observed that the DC current increases more than when operating under normal (50-Ohm load) conditions. The increase of DC current, under load mismatch conditions, is highly undesirable (particularly in a hand-portable environment) as it causes increased power consumption and may over-load the power amplifier transistor device thereby resulting in damage to, or failure of, the device.
Known mechanisms do not solve the aforementioned problems. For example, voltage limiters do not protect the PA from mismatch conditions that lead to high current. Furthermore, existing current limiters do not protect efficiently the PA from battery voltage variations under mismatch. Known solutions use circuitry external to the power amplifier module to realize the function, when the PA is located on Gallium Arsenide (GaAs). Undesirably, this results in extra inputs/outputs (I/O) leads on the PA die. Alternatively, if a monolithically integrated circuit (IC) is used, for example where the PA is manufactured on Silicon Germanium (SiGe), approximately 20% of additional die size is required.
To accommodate mismatch problems, protection circuits are often used. A standard current limiter protection circuit is illustrated in FIG. 1. In FIG. 1, the radio frequency input signal (RFin) 105 is input to a base port of power transistor 110. The power transistor 110 is supplied from a battery voltage 115 via an RF choke inductor 120 to provide a RF amplified output voltage 125.
The emitter port of the RF power transistor 110 includes a sensing resistor 130 to ground. The emitter port of the RF power transistor 110 is also operably coupled to a protection circuit 140. The protection circuit 140 comprises a multiplier circuit 145, receiving the detected voltage developed across the sensing resistor 130 and a comparator circuit 150, comparing the output from the multiplier circuit 145 with a reference voltage 155, which sets the chosen limiting current. The comparator circuit 150 output is input to a transistor (Q1), which effectively is switching the bias circuit 135 to ‘ground’ when the voltage coming from the multiplier 145 is greater than the reference voltage 155, thereby reducing the bias current of the power transistor 110.
Thus, as shown, the use of a protection circuit adds significantly to the size and complexity of power amplifier circuits, with regard to the extra circuitry/components (typically two operational amplifiers) that are required to generate/compare the detected voltage to the reference voltage 155, which is set by additional external circuitry.
However, the effectiveness of the protection circuit is compromised due to the operational amplifier offset voltage ε1 (comparable to the detected voltage Vdet) and to the variation of Vdet with regard to sensing resistor 130 dispersion. It is noteworthy that a low value resistor is more sensitive to process variation than a high value resistor. In addition, the collector efficiency of the power transistor 110 is degraded by the sensing resistor 130. Furthermore, the complexity of the protection circuit may typically lead to undesirable loop stability issues, due to high loop gain.
Thus, a need exists for an improved RF device, such as a wireless communication unit, RF PA module and method of operation therefor, which prevents high current under extreme VSWR conditions, wherein the aforementioned problems with prior art arrangements are substantially alleviated.